Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmia generally includes supraventricular tachyarrhythmia and ventricular tachyarrhythmia. Fibrillation is a form of tachyarrhythmia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial (SA) node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. Ventricular tachyarrhythmia occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles or when a biologic pacemaker (focus) in a ventricle usurps control of the heart rate from the SA node. When the atria and the ventricles become dissociated during ventricular tachyarrhythmia, the ventricles may contract before they are properly filled with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.
Ventricular cardioversion and defibrillation are used to terminate most ventricular tachyarrhythmias, including ventricular tachycardia (VT), and VF. An implantable cardioverter defibrillator (ICD) is a CRM device that delivers cardioversion/defibrillation pulses, each being an electric shock, to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory. An ICD typically also delivers another type of electrical therapy for tachyarrhythmia known as anti-tachycardia pacing (ATP). In ATP, the heart is competitively paced in an effort to interrupt the reentrant loop causing the tachyarrhythmia.
In an ICD that includes ATP and cardioversion/defibrillation capabilities, the efficacy of each available anti-tachyarrhythmia therapy depends on, among other things, origin of the tachyarrhythmia. For example, a ventricular anti-tachycardia therapy is generally ineffective in terminating an atrial tachyarrhythmia. Additionally, the delivery of each cardioversion/defibrillation pulse consumes a considerable amount of power and results in patient discomfort owing to the high voltage of the shock pulses. Therefore, for therapy efficacy, device longevity, and patient satisfaction, among other things, there is a need for accurate classification of each detected tachyarrhythmia episode.